The need to control the world's greenhouse gases is a principle focus of the world today. Greenhouse gases, i.e. carbon dioxide, may be emitted into the atmosphere through natural processes and human activities, such as the combustion of fossil fuels (oil, natural gas, and coal), solid waste, trees and wood products, and also as a result of other chemical reactions (e.g., manufacture of cement). Carbon dioxide is a particularly critical greenhouse gas because it not only transmits visible light, but strongly absorbs energy in the infrared wavelengths at which the earth radiates energy to space. The absorbed energy may be re-radiated to the earth, thereby warming the earth. Atmospheric mixing ratios for carbon dioxide are now higher than at any time in the last 800,000 years, standing at 383 parts per million (ppm) compared to a pre-industrial revolution high of 280 ppm, although this value may vary by location and time. Currently, carbon dioxide emissions in the U.S. are at about 6 billion tons annually and 18 billion tons globally. Reducing carbon dioxide emissions from its source is one primary aim of a number of global warming protocols, however, substantial elimination of carbon dioxide production from the vast majority of CO2 emission sources is not likely to be realized.